p-Aminohippurate Transport in the Proximal Straight Tubule: Development and Substrate Stimulation

Abstract

Summary pAminohippurate transport by the developing kidney has previously been shown to increase as a function of age and to be enhanced by pretreatment with penicillin. The relative contribution of increases in intrinsic cellular transport capacity and in tubular length was assessed in isolated perfused proximal straight tubules obtained from developing rabbits. The intrinsic capacity for transpurt (10-l5 M/min -mm) increased 4.8-fold (y = -315 + 62.0x, r = 0:69, P < 0.005) from 8-19 days of age. Tubular length (microns) also increased (y = -38.5 + 52.3x, r = 0.84, P < 0.001) during the same period. Nine-to 13-day-old animals pretreated with procaine penicillin (400,000 U) divided over 3 days had an intrinsic capacity for transport of 684 f 88 x 10-l5 M/min mm, which was 89% greater than the 361 f 84 x 10-l5 M/min mm observed in age-matched, untreated controls ( P < 0.02). In these groups tubular length was 630 106 pm and 585 f 37 pm, SpeculationMost of the increase in absolute transport (intrinsic capacity x tubular length) seen with age is the result of the increment in the intrinsic transport capacity, while 33% is a consequence of the rise in the surface area of the transport membrane. Substrate stimulation augments the rate of maturation of the intrinsic transport mechanism without altering the age-related change in the length of the proximal straight tubule.Uptake ofp-aminohippurate (PAH) by renal cortical slices and secretion of this substance by the kidney have been shown to rise during development (7, 11,15, 21,24,25). This may result from increases in intrinsic tubular transport capacity, the ratio of mass of the transporting segments to cortical mass, or both. Pretreatment of young animals with penicillin hastens the increase in uptake or in excretion of PAH, possibly through similar mechanisms (8,9,15). None of the methods thus far used can distinguish between these alternatives. Moreover, they are subject to a number of limitations. For instance, clearance measurements are influenced by the developmental changes in the intrarenal distribution of blood flow (2, 13,22), which result in alterations in the delivery of substrate to the secreting segments of differing nephron populations; the slice technique detects only the intracellular accumulation of PAH and as a result gives information which corresponds only tenuously to net transepithelial transport (28). In addition, both methods relate transport to the weight of the tissue studied, yet the proximal tubule, the nephron segment responsible for PAH transport, comprises only a finite portion of the cortical or whole kidney mass. The isolated perfused tubule does not suffer from these limitations. Furthermore, it enables one to identify and measure the individual contribution made by changes in tubular length and intrinsic capacity to the maturation of the renal trans---of organic anions. MATERIALS A N D METHODSSixteen suckling New Zealand White rabbits, aged 8-19 days, were studied for the changes in PAH transport with age. The effect of substrate s...

Acid incubation reverses the polarity of intercalated cell transporters, an effect mediated by hensin

“…Equilibration, transport, and cell pH studies were performed using Burg's solution in the perfusate and bath, containing (in mM) 120 NaCl, 25 NaHCO 3 , 2.5 K 2 HPO 4 , 2 CaCl 2 , 1.2 MgSO 4 , 5.5 D-glucose, 1 trisodium citrate, 4 sodium lactate, and 6 L-alanine, 290 ± 2 mOsm/kg H 2 O and gassed with 94% O 2 -6% CO 2 , yielding a pH 7.4 at 37°C (3,5,20). The bath was continually exchanged, except when precious Ab's were used during 3-hour incubations; in those cases, osmolality was maintained within ±1% by adding 240 µl water per hour (22) and replacing the bath and Ab's each hour. Incubations for 3 hours in acid (pH 6.8 in both luminal and bathing solutions) and control (pH 7.4) media were described previously (3,16,20,23); incubation at pH 6.8 yields a physiology comparable to 3 days of acidosis in vivo, while that at pH 7.4 sustains net HCO 3 -secretion.…”

Acid incubation reverses the polarity of intercalated cell transporters, an effect mediated by hensin

“…(3,5,20). The bath was continually exchanged, except when precious Ab's were used during 3-hour incubations; in those cases, osmolality was maintained within ±1% by adding 240 µl water per hour (22) and replacing the bath and Ab's each hour. Incubations for 3 hours in acid (pH 6.8 in both luminal and bathing solutions) and control (pH 7.4) media were described previously (3,16,20,23); incubation at pH 6.8 yields a physiology comparable to 3 days of acidosis in vivo, while that at pH 7.4 sustains net HCO 3 -secretion.…”

Developmental Aspects of Sugar Transport by Isolated Dog Renal Cortical Tubules

“…However, this rise in length and surface area of the proximal tubule with development does' not appear to account completely for the increased transport capacity. Indeed, Schwartz et al (29) have estimated that only one-third of the rise in the tubular transport of p-aminohippurate can be ascribed to an increase in tubular surface area, while two-thirds of the increase is due to an enhanced intrinsic transport function of the proximal tubule cell.…”